IIT

Check out the official website for previous year papers:
http://jeemain.nic.in/webinfo/QuestionPapers2014.htm

There is no preferred criteria. Both the criteria are valid, you need to fall under any of the two to be eligible for the IITs and the NITs.

Hello Kartic,
It's not space Science to study for JEE Mains. It's actually quite simple & straight forward. Assign time for each subject every day. Assign some time for exercise everyday. Take mock tests. Get results evaluated to know weak areas and give more time for study of weak areas. If you follow this, then I am sure you will do well.

Hello Ayush,
There is no standard yardstick to measure performance of these coaching institutes. Authenticity of the adverts placed by these institutions in newspapers is unverified. Hence one had to take it with a pinch of salt. Having said this, it still remains open as to where to go. I will advise you to select that institute in which you have more faith.

IIT is the best. BITSAT is the second choice.

No, BITS exam is easier than IIT. IIT exam is tough that every year around 15 lakhs candidate appear for it. The number of participants is directly proportional to toughness of the exam.

NIT and IIT provide admission by through JEE Mains and Advance exam. Therefore, appear for the same to get admission in IIT or NIT.

To get admission in IIT is tough and only bright students can get admission in IITs. IITs follows relative grading system for academic performance evaluation. Desire to excel and scoring high grades puts students under constant pressure, but it help students prepare for competitive world.

Below mentioned are the top colleges:
1. BITS – Pilani.
2. Punjab Technical University.
3. Amity University.
4. Thapar University.
5. Sikkim Manipal.
6. Techno India.
7. Jadavpur University.
You can check the below mentioned link:
https://www.shiksha.com/top-engineering-colleges-in-india-rankingpage-44-2-0-0-0

All those who are passing 12th or equivalent from recognised boards are allowed. List of such institutes is available on websites of JEE Mains.